It is desirable, and often necessary, to determine the cardio-respiratory function in human subjects. The function of the cardio-vascular and respiratory system is to supply oxygenated blood to the body tissues and to remove the CO.sub.2 produced by the tissues for excretion by ventilation of the lungs. The amount of blood pumped or vented and the amount of O.sub.2 and CO.sub.2 in the blood, as well as the volume of lung ventilation, are critical reflections of the adequacy of the circulatory and respiratory function.
During exercise, during disease states, or during surgery, these physiological parameters are adaptively altered and may be impaired. In order to diagnose and treat cardio-respiratory dysfunction, it is necessary to measure and evaluate these parameters. This is particularly true during surgical anesthesia, where the anesthetist must maintain cardio-respiratory homeostasis that may become impaired from the anesthetic agent or from complications during surgery. It is also necessary to assess these parameters in critically ill non-surgical patients while being managed in critical care units. Moreover, assessment of cardiac output and other cardio-respiratory functions, which should be a key part of exercise testing, is not evaluated routinely because there are presently no effective non-invasive methods which are acceptable.
Invasive systems are available, but cannot be routinely used because the insertion procedure (via catheter or the like) is time-consuming and involves risk. Non-invasive devices, such as the ultrasonic Doppler device, have been developed, but cannot be used routinely and are unable to continuously accurately determine the cardio-respiratory function.